1. Field of the Invention
This invention relates in general to apparatus and method for locating short-circuits in multi-layer circuit boards.
2. Description of the Prior Art
In large electrical devices, a number of individual component assemblies are provided in which permanent circuit paths are provided on copper-clad insulated boards which are stacked together and connected by plated-through contacts which extend from one board to the others. Multi-layer circuit boards of this type include boards comprised solely of conductor paths for series or shunt electrical connections and of those which provide earth paths or voltages on the entire surfaces. Such conductor paths are produced by etching the film so as to provide the desired conductor path for the circuits and for the ground and voltage planes and through contacts are connected at selected points to interconnect the circuits on the boards.
In order to assure that the assembled multi-layer package does not become too thick since multi-layers having up to 30 layers are used, the individual layers are formed from thin copper-clad foils which might be approximately 0.2 mm. Electrical faults which are observable which occur in the production of the multi-layer circuits such as etching bridges and breaks in the conductor paths exist as well as additional faults which arise during the connecting together of the individual multi-layer levels. Thus, in the process of stacking and bonding the various boards together, faults can result due to the fact that a layer may become distorted such that undesired connections may take place during the installation of the through contacts. It is also possible for undesirable short-circuit connections to be caused by splashes of tin during surge soldering.
It is therefore imperative that multi-layer circuit boards be checked for possible faults; and in large production quantities of stack circuits, computers can be used for this purpose. However, at times, small production quantities are produced of stacked boards, as for example, when customers ask for samples in small quantities for experimental or test runs and the use of computers for testing such small quantity production boards is too expensive. Thus, for testing small quantities of stack boards checks of random specimens might be made and faults in the boards may not be noticed until the boards are installed in the operating apparatus.
The cost of producing small numbers of multi-layer boards is much more expensive than the cost of individual boards made in production quantities and often times spare units are not made in sample quantities and it is difficult to repair the faulty unit. When a discontinuity exists in conductor paths which should be electrically connected, it is possible to bridge the discontinuities with a wire conductor and thus eliminate the circuit interruption. However, short-circuits occur more frequently; and it is much more difficult to eliminate short-circuits particularly due to the fact that short-circuits may occur at internal locations in the stacked assembly of the boards and the location of the short-circuit cannot be visually observed from the exterior of the component. A crude method of eliminating short-circuits exists wherein a surge of current is applied to the shorted leads, however, by burning the short-circuit out in this manner leads to distruction of the multi-layer unit because the gases which are formed due to the burn-out current cause the multi-layer circuit board to be inflated.